Some present day winch systems for controlling tension on a mooring line employ a pair of parallel traction drums and a storage drum, where the rope coming from the load is passed a multiple times around the pair of traction drums and then guided to the storage drum. The traction drums hold the rope by friction and operate as the principal power for pull-in means or braking means for paying out line, whereas the storage drum upon which the low tension end of the line is spooled, supplies the tension required to maintain the frictional forces between the rope and the traction drums. Maximum holding capacity is thus limited to the friction established between the contacting surfaces of the rope and the sheaves/pulleys on the drum and the tension load supplied on the low load side of the winch. The rope tensioning will be distributed over the axial contacting area of the winch until force equilibrium has been obtained.
However, during pull-in or paying-out of the rope there are other parameters that must be taken into account to maintain optimal yield capacity of the winch.
When the rope enters the winch at high tension, and hence a large degree of stretching, the rope tensioning should ideally be significantly reduced when passing the first two or three sheaves, thereby reducing the degree of stretching. The result is that, per time unit, the amount of rope entering and leaving a sheave is not identical causing a micro-skidding between the rope and the sheave, i.e. skidding that does not cause a net translational movement of the rope relative to the underlying sheave. Hence, given a certain sheave diameter of this initial, micro-skidding sheave and a rope having a certain Young's modulus, there exists an ideal sheave diameter of the subsequent sheave of the winch that sustains an optimum winching capacity.
For example, if the sheave diameter of the subsequent sheave is larger than the ideal sheave diameter, this sheave will require more rope to avoid skidding. Hence, the reduction of the rope tensioning becomes less than the maximum reduction causing the tensioning to propagate further towards the low load side of the winch. Calculated from the low load side it is possible to find a maximum available counter tensioning for each sheave which depends on the applied low side tensioning and the contact surface friction between the sheave and the rope. If this maximum available counter tensioning is not sufficient to balance the tensioning from the high load side of the winch the result will be a continuous skidding of the rope.
On the other hand, if the sheave diameter of the subsequent sheave is smaller than the ideal sheave diameter, this sheave will require less rope to avoid skidding. This is clearly not possible since the reduction of tensioning over the initial sheave cannot be less than the sheave's maximum force transmission capacity. Therefore, the subsequent sheave receives an excess amount of rope, causing a sudden tension reduction. As a consequence there will not be sufficient counter tensioning to balance the load on the high load side of the initial sheave, causing a continuous skidding over the latter. If the mismatch in diameter continues the result would be that the rope is continuously loosing the tensioning towards the low load side of the winch.
Another important challenge occurs during operation of a traction winch at very low loads. In this situation it is not certain that the any skidding will take place on the first sheaves on the high load side. The result may be piling of rope on the winch which again causes the rope to be suspended underneath the drums at one or more turns. Except from being a problem in itself, a rapid change in load could cause skidding over an extensive length at high velocity, thus increasing the risk of damages.
In general, extensive skidding of a rope/cable on a winch must be avoided since skidding causes wear. This is of particular importance at high load.
Hence, in modern traction winches these well known challenges have normally been solved by finding a compromise to ensure that a certain rope/cable having a certain load works in an optimized manner.
The above mentioned challenges are particularly evident when mooring elastic cables such as synthetic ropes under high tension since the level of compensation due to elastic contractions and elongations of the rope as the rope tension diminishes and increases, respectively, while passing through the winch is particularly high.
During the last decades several solutions have been suggested to meet these challenges. An example of publication addressing the challenge of compensating contraction/elongation of ropes is found in FR 1,105,165 disclosing as solution involving decrease in sheave diameter from the high tension side of the drums to the low tension side. Furthermore, U.S. Pat. No. 7,175,163 discloses a winch wherein the sheaves, or at least the part of the sheaves contacting the cable/rope, is made of a product that is sufficiently elastic to follow any changes in the cable length due to high load, while at the same time maintain high friction between the contacting surfaces.
However, a disadvantage of this prior art publication is a poor capacity to quickly and simply adjust to cables having significantly different contraction and elongating properties during operation. One example is the replacement of traditional fibre ropes with relatively high elasticity (common Young's modulus 1-1.4 GPa) with high yield fibre rope such as high yield polyethylene fibre (common Young's modulus: 35-45 GPa), thus reducing the longitudinal stretching significantly at identical loads. In addition, such high yields fibre ropes have much lower frictional coefficients with steel, increasing the possibility of skidding on the underlying sheave/pulley.
U.S. Pat. No. 3,966,170 and GB 1,387,493 discloses a solution involving dissimilar rotation velocity of the drums, resulting in a fairly complex and expensive system.
None of the prior art publications discloses a solution in which the winch may be reconfigured to optimize the suitability for ropes/cables with Young's modulus in both low and high ranges, for example traditional fibre ropes and high yield fibre ropes, respectively.